In digital communication systems, information, such as data or voice, is carried in an encoded form on a communication channel. In a time division multiplexing (TDM) communication system the communication channel is a time slot or frame into which a burst of encoded information is inserted. In addition to the information being communicated, the burst will also necessarily include additional encoded information, such as header information, control information, synchronization, training or optimization information, and the like, as is well known in the art. On the receiving end, the information burst is decoded to recover the communicated information. If the information is not decoded correctly, errors occur. The rate at which errors occur in the decoded information is referred to as the bit error rate (BER). A related measure of decoded information reliability is the block error rate (BLER) of a group of information bits, where a block error is defined as one or more bit errors within the group of information bits. Reducing the BER or BLER enhances system performance by, for example, increasing the amount of data that may be communicated or, for voice communications, enhancing the perceived quality of the voice communication.
In an equalized communication system that transmits bursts of information over a time varying channel, where a burst consists of one or more known sequences of training symbols (encoded and modulated information used for optimizing the equalizer) and unknown data and header symbols (i.e., encoded and modulated information), the channel BER profile is generally a function of the coded symbol position. For example, the channel BER of Enhanced Data Rates for GSM Evolution (EDGE) Enhanced General Packet Radio System (EGPRS) is higher at the beginning and the end of the information burst than near the middle of the information burst where the training symbols are located. Furthermore, the BER of every third bit is roughly twice the BER of the first and second bits. This "symbol asymmetry," due to the bit-to-symbol mapping scheme for Gray-coded 8-PSK (phase shift keying) used in EGPRS, occurs for many constellations and bit-to-symbol mappings.
The header symbols (throughout the term "header" is used to refer to a variety of header and control information including, without limitation stealing bits, coded USF (uplink status flag), coded RLC/MAC (Reliable Link Layer/Medium Access Control) header information, and the like) typically carry crucial information. Errors within the data symbols may result in slightly decreased data rates or slightly degraded voice quality. Errors in the header symbols can result in a loss of the entire information burst, which can result in significantly decreased data rates or significantly decreased voice quality. Therefore, communication of the header symbols should be made as reliable as possible.
One method for minimizing the decoded BER of the header symbols includes placing the header symbols adjacent to the training sequence. This method results in minimizing the interleaver gain while maximizing the effect of the BER profile as described above. Another method includes interleaving the data and header together. This method results in maximizing the interleaver gain while minimizing the effect of the BER profile. Neither method is superior for all channel conditions, and each method may cause a significant degradation to the data decoded BER.
Thus, there is a need for a method and apparatus for mapping symbols to an information burst that minimizes header decoded BER over many channel conditions without degrading data decoded BER.